The invention relates to a tire having a least one component comprised of a rubber composition which contains aggregates of fumed silica reinforcement. Such tire component may be a tire tread.
Tires are conventionally prepared with components of various rubber compositions which contain particulate reinforcement in form of carbon black and/or amorphous silica.
While the amorphous silica reinforcement for such tire component rubber compositions are conventionally in a form of precipitated silica, it is to be appreciated that silica such as pyrogenic, or fumed, silica has sometimes also been suggested for such purpose.
As reported in the Handbook of Fillers for Plastics by H. S. Katz, et al (1987), published by Van Nostrand Reinhold Company, Pages 174 through 177, fumed silicas are fluffy white powders of an amorphous structure which may be produced, for example, by hydrolysis of silicon tetrachloride in a flame of hydrogen and oxygen at a temperature of, for example, about 1100xc2x0 C. or 1800xc2x0 C. in which molten spheres of fumed silica are formed (primary particles) which collide and fuse with one another to form three-dimensional, somewhat branched, chain-like particles (secondary particles) which may be referred to as aggregates.
Fumed silicas can be modified according to the above publication by reaction with silanol groups or by altering the flame process. Commercially available fumed silicas, according to the publication, may be divided into three groups, namely hydrophilic fumed silicas, hydrophobic fumed silicas and mixed oxide fumed silicas. The exemplary hydrophobic silica is created by reacting a hydrophilic silica with an organosilane such as, for example, dimethylchlorosilane.
Fumed silicas typically have smaller bulk densities (uncompacted) than precipitated silicas. For example, fumed silicas reported in the above publication in 1987 had associated bulk densities (uncompacted) ranging, for example, from 37 to 130 g/l. For example, representative Aerosol fumed silicas from Degussa in are reported as having bulk densities ranging from 50 or 130 g/l. Representative Cab-O-Sil fumed silicas from Cabot are reported as having bulk densities of 37 and 72 g/l. Representative HDK fumed silicas from Wacker are reported as having a bulk density of 60 g/l.
In the Encyclopedia of Chemical Technology Third Edition, Vol. 20 (1982) by Kirk-Othmer, Page 768, it is reported that a bulk density of pyrogenic silica may range from 30 to 1200 g/l and that a bulk density of precipitated silica may range from 30 to 3000 g/l.
In the description of the invention, the term xe2x80x9cphrxe2x80x9d relates to parts by weight of a particular ingredient per 100 parts by weight of rubber contained in a rubber composition.
The terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d are used interchangeably unless otherwise indicated, and the terms xe2x80x9ccurexe2x80x9d and xe2x80x9cvulcanizexe2x80x9d may be used interchangeably unless otherwise indicated.
The invention relates to a tire having at least one component comprised of a rubber composition which contains aggregates of fumed silica reinforcement.
In accordance with this invention, a tire is provided having at least one component of a rubber composition comprised of, based upon 100 parts by weight of elastomer (phr),
A. 100 phr of at least one diene hydrocarbon based elastomer,
B. about 35 to about 100 phr of particulate reinforcement comprised of about 35 to about 100, alternately about 35 to 90, phr of a branched, fumed amorphous silica and from zero to about 65, alternately about 10 to about 65, phr of at least one additional particulate reinforcement selected from of carbon black, synthetic amorphous silica and silica treated carbon black, wherein said fumed silica aggregates contain hydroxyl groups (e.g. silanol groups) on their surface; wherein prior to mixing with said elastomer(s), said fumed silica aggregates have a pour density in a range of from about 40 to about 60 g/l and a specific density in a range of from about 3.8 to about 5 g/cm3; and wherein said filmed silica aggregates are
(1) blended with said elastomer(s) together with a silica coupler having a moiety reactive with said hydroxyl groups on the surface of said fumed silica and another moiety interactive with at least one of said diene-based elastomers, preferably an organosulfur silane of the general formula (I)
[Xm(R2O)3xe2x88x92mSixe2x80x94R3]2S2xe2x80x83xe2x80x83(I)
xe2x80x83wherein X is a halogen radical, preferably a chlorine radical, R2 is an alkyl radical having from 1 to 3 carbon atoms, preferably selected from methyl and ethyl groups; R3 is an alkylene radical having from 1 to 18 carbon atoms, preferably from 2 through 4 carbon atoms; m is zero or an integer of from 1 through 3, preferably zero or an integer of 1 or 2; and z is an integer of from 2 to 8, preferably an average of 2 to about 2.6 or from about 3.5 to about 4, or
(2) pre-treated with an alkylsilane of the general formula (II) and thereafter blended with said elastomer(s) and said silica coupler:
Xnxe2x80x94Sixe2x80x94R4xe2x88x92nxe2x80x83xe2x80x83(II)
xe2x80x83wherein R is an alkyl radical having from 1 to 18 carbon atoms, preferably from 1 through 4 carbon atoms; n is an integer of from 1 through 3; X is a radical selected from the group consisting of halogens, preferably chlorine, and alkoxy radicals having from 1 through 3 carbon atoms, preferably selected from methyloxy and ethyl radicals; or
(3) pre-treated with at least one material prior to blending with said elastomer(s) selected from at least one of
a. hexamethyldisilazane;
b. an organosulfur silane of said general formula (I);
c. an organosulfur silane of the general formula (III)
(X)m(R2O)3xe2x88x92mSixe2x80x94R3 xe2x80x94SHxe2x80x83xe2x80x83(III)
xe2x80x83wherein X is a halogen radical, preferably a chlorine radical; R2 is an alkyl radical having from 1 to 18 carbon atoms, preferably selected from methyl and ethyl groups; R3 is an alkylene radical having from 1 to 18 carbon atoms, preferably from 2 through 4 carbon atoms; m is zero or an integer of from 1 through 3, preferably zero or an integer of 1 or 2; and z is an integer of from 2 to 8, preferably an average of 2 to about 2.6 or from about 3.5 to about 4; and optionally in addition to the above materials.
d. an alkylsilane selected from said general formula (II).
In one aspect of the invention, said fumed silica is pre-treated with a combination of:
A. a material of the general formula (I) and of the general formula (II), and
B. a material of the general formula (II) and of the general formula (III),
In the practice of this invention, where said fumed silica is pre-treated with at least one material of the general formula (I) or (III), preferably said pre-treated fumed silica is blended with said elastomer(s) in the absence of said silica coupler unless an amorphous precipitated silica, silica treated carbon black or fumed silica which has not been so pre-treated is added to said elastomer(s) and where it is desired to blend a silica coupler with said elastomer on behalf of said un-pretreated silica materials.
A significant consideration of use of the specified branched fumed silica for reinforcement of a rubber tire component is its inherent substantial absence of water (about 0.1 to about 1.5 weight percent water), prior to mixing with the elastomer(s) as compared to a conventional synthetic amorphous precipitated silica (containing about 3.5 to about 8 percent water). This is considered herein to be significant because the relative lack of coordinated water on the surface of silica makes the silica surface more accessible to surface treatment, including said pre-treatment specified by this invention.
The fumed silica contemplated for use in tire components for the present invention is characterized by a relatively visibly branched, less than spherical, structure as opposed to fumed silica which has a relatively visibly more spherically oriented, although still somewhat branched structure. Apparently as a result of the more visibly branched structure, the fumed silica for the tire components of the present invention exhibits relatively different pour densities and specific densities than such other fumed silica.
The branched fumed silica for the tire components of this invention has a pour density (ASTM D1513), which may also be referred to as its apparent density, ranging from 40 to 60 g/l prior to mixing with the elastomer(s). This is intended to be differentiated from fumed silicas having a pour density ranging from 50 to 160 g/l. By comparison, precipitated silicas (not a fumed silica) typically have a pour densities ranging from about 280 to about 350 g/l. There are therefore clear and considerable differences in the respective apparent, or pour, densities.
The branched fumed silica for use in the tire components for this invention also has a specific density (ASTM D2638) in a range of from 3.8 to 5 g/cm3, prior to mixing with the elastomer(s), in contrast to fumed silicas having a specific density in a range of about 2.7 to 3.5 g/cm3. By comparison, precipitated silicas typically have a specific density in a range of from 2.1 to 2.4 g/cm3. There are therefore clear and considerable differences in the respective specific densities.
A significant aspect of the invention is that the fumed silica aggregates for this invention are required to be characterized by both the pour (or apparent) density and the specific density characteristics prior to their blending with the elastomer(s) rather than to be characterized by a pour density or a specific density alone. It is considered that the very considerable differences between the pour density and the specific density of the required fumed silica aggregates is an indication of a considerable degree of branching thereof. Accordingly, it is considered herein that the required fumed silica aggregates for use in this invention are necessarily required to be characterized by both the pour and specific densities
The branched fumed silica aggregates for the tire components of this invention are generally of an elongated, branched form comprised of a primary trunk composed of fused primary silica particles having an average length to diameter (L/D) in a range of about 1.1/1 to about 4/1 and having a multiplicity of branches composed of primary fused silica particles with an average length to diameter (L/D) in a range of about 1.1/1 to about 4/1.
As a result of the structure of such fumed silica aggregates, such fumed silica aggregate may be preferentially oriented in the rubber composition for a tire component and therefore present unique reinforced tire components. For example, it is contemplated that from about 20 to about 80 percent of such fumed silica aggregates may be oriented in substantially the same direction in the rubber composition of the tire component, to thereby result in anisotropic rubber reinforcement. Such orientation may be achieved by processing the fumed silica reinforced rubber component prior to assembly of the tire, such as by extrusion or directional milling.
In one aspect of the invention, it is preferred that the branched aggregate of fumed silica particles for this invention is characterized by having a BET-SSA surface area in a range of about 100 to about 400, preferably about 150 to about 350, m2/g; and a CTAB surface area in a range of about 100 to about 400, preferably about 140 to about 350 m2/g, wherein the ratio of said BET and CTAB surface areas is in a range of about 0.9 to about 1.2.
The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849-80 for set-up and evaluation. The CTAB surface area is a well known characterization of silica.
The BET surface values may be obtained, for example, by using the method described in Brenner, Emmett, Teller Journal of the American Chemical Society, Volume 60, Page 309 (1938). Set-up conditions may be suitably described as using a 500 mg of sample; first degassing 2 hours at 105xc2x0 C., ambient pressure; and a second degassing for 1 hour at 150xc2x0 C. at 10 to 3 mm Hg. The BET surface area is a well known characterization of particles and aggregates of various materials including silicas.
A branched fumed silica which may be used for reinforcement of the tire component of this present invention is commercially available from Wacker Chemie GmbH under the designations S13, V15, N20, T30, T40, H15, H20, H30, H2000, H2000/4, H3004, H2015EP and H2050EP. The silicas under the designations S13, V15, N20, T30 and T40 are considered herein as xe2x80x9cuntreatedxe2x80x9d and fall within the above described density ranges. The silica under the described designations HI5, H20, H30, H2000, H2000/4, H30004, H20/SEP and H20S0EP are considered herein as being xe2x80x9ctreatedxe2x80x9d with surface modifiers. For example, H20 is understood to be the N20 fumed silica further modified with dichlorodimethylsilane. The H2000 silica is understood to be the N20 fumed silica further modified with hexamethyldisilazane.